Double cladding fibers (DCF's) offer a practical means for constructing high power fiber amplifiers and fiber lasers. In DCF's, pump light propagates in a large (typically 50–500 μm diameter) inner cladding that surrounds a smaller (typically 3–20 μm diameter) core that can doped with the active dopant such as Er, Yb, Tm, Nd, or Pr. The inner cladding is surrounded by a low refractive index outer cladding and functions as a low loss waveguide for the pump light. The relatively large inner cladding allows high power, low spatial coherence pump sources such as broad area lasers, diode bars or fiber-coupled diode bars, to be efficiently coupled into it. The power that can be coupled from such sources into a fiber is proportional to aNA2, where a is the fiber inner cladding cross-sectional area and NA is inner cladding numerical aperture. The power can be maximized by reducing the refractive index of the outer cladding to increase NA, by using low refractive index polymers, resulting in NA's of 0.35–0.6.
Four primary methods for coupling of light into DCFs currently are used: i) end-fire, with pump light directly coupled through the end of the DCF, ii) tapered fused fiber coupler, with the pump light coupled through a multimode fiber fused to the DCF, iii) tapered fiber bundle, with pump light coupled through multiple fibers fused to the DCF, and v-groove side-coupling, with the pump light coupled into the DCF by total internal reflection (TIR) from a 45° facet of a v-groove cut into the inner cladding.
V-groove coupling is typically implemented as shown in FIG. 1. The DCF fiber is first mounted on a transparent substrate (for mechanical strength) using a transparent, low refractive index adhesive, which required to maintain optical guiding in the inner cladding. A micro-lens collects the widely diverging laser diode emission, and then re-focuses the emission onto the 45° v-groove facet, where TIR at the air-glass boundary couples the light into the fiber. To assure pump capture in the inner cladding, the incident cone of light from the micro-lens must have a numerical aperture that is smaller than the NA of the DCF. While in the configuration of FIG. 1 the broad stripe diode is oriented with the stripe parallel to the fiber axis, diode orientation with the stripe perpendicular to the axis can also be used.
Compared with the other methods, the v-groove technique offers the advantages of small size, negligible loss for the signal in the core, low cost, and very high pump coupling efficiency. Small size is particularly important because it allows the integration of a pump diode and the DCF coupling means into a single compact package, which can be hermetically sealed. V-groove coupling, however, also has significant deficiencies, limiting V-groove coupling use for many commercial applications. These deficiencies are described below.
1. Requirement for a Low Refractive Adhesive: Poor Mechanical Strength.
This represents a significant limitation since all optically transparent low refractive index optical adhesives have low mechanical strength. All such adhesives are in the form of soft polymers or silicones that do not produce a strong bond between the fiber and the glass substrate, creating the possibility that the fiber will move, causing a drop in pump coupling efficiency.
2. Requirement for a Low Refractive Adhesive: Low Optical Damage Level.
Focused pump light propagates through the low refractive index adhesive, subjecting the adhesive to a high optical intensity. Since optical damage levels for such adhesives are orders of magnitude lower than glass, optical damage to the adhesive can occur. Such damage can be immediate, causing burning of the adhesive and damage to pump diode and micro-lens, or can occur over a long time leading to a slow decrease in the transmission and pumping efficiency, eventually producing a runaway pump absorption effect causing complete destruction of the v-groove.
3. Limited Tolerance for Positioning of the Diode and the Micro-Lens.
A large distance from the laser diode facet to the v-groove requires that the micro-lens magnification M (approximately equal to the ratio of lens-to-focused-spot distance divided by the lens focal length) be large (typically 10×). As a result, any change in the diode or lens lateral position results in an M times larger change in the focused spot position, leading to a very limited alignment tolerance for the diode and the lens. Although increasing the lens focal length reduces M, increasing the lens focal length requires the lens overall size be increased significantly, thereby increasing the overall package size.
4. Unsuitability for Lens-Less Coupling.
By eliminating the high precision micro-lens, the cost of a side-coupling arrangement can be reduced significantly not only because of elimination of this costly element, but also because the alignment procedure for two elements (diode and fiber) is significantly simpler and easier to automate than the alignment procedure for three elements (diode, lens, and fiber). In addition, the elimination of the high magnification lens significantly increases the alignment tolerance, leading to a lower cost and more robust pump diode-DCF pump coupler package. Unfortunately, v-groove coupling is unsuitable for lens-less coupling.
The unsuitability of prior art v-groove couplings for lens-less side coupling can be explained with reference FIG. 2. With no lens, the diode is placed in close proximity to the top surface of the DCF, as shown in FIG. 2. The minimum distance h between the diode facet and middle point of the v-groove facet is given by the sum of the clearance distance between the substrate and the diode facet, substrate thickness t, adhesive layer thickness, and approximately ¾ of the fiber diameter. To provide sufficient mechanical strength, t is typically >200 μm. Assuming a diode-fiber clearance distance of 20 μm (to insure that the substrate does not contact the diode) adhesive thickness of 10 μm, a substrate thickness of 200 μm, and a typical fiber diameter of d=125 μm, the total diode-to-v-groove distance is h=323 μm. If a broad area laser diode with a stripe width of w=50 μm and a divergence of 12° (8° in glass) in the junction plane is used, the pump beam will expand to approximately a width of 97 μm at the mid-point of the v-groove facet. For a 90° v-groove with a 52 μm vertical depth, the maximum allowed vertical depth in such a fiber to avoid penetrating into the evanescent field of a 5–10 μm diameter core, the v-groove facet width is 52 μm. Therefore, the 97 μm wide beam significantly spills over the v-groove facet, lowering pump coupling efficiency. While a larger fiber diameter allows a larger v-groove width, the larger fiber diameter undesirably decreases the pump absorption in the fiber, in turn, undesirably requiring a longer fiber to fully absorb the pump light. Additionally, the smaller pump intensity resulting from such a larger cladding reduces population inversion of the active dopant, leading to larger thresholds and less uniform gain distribution.
Although one example of a v-groove coupling seen in FIG. 6 of U.S. Pat. No. 5,854,865 does allow lens-less coupling with a laser diode placed above the open side of the v-groove, this example does not rely on TIR. Instead, to achieve high reflectivity, this example requires that the v-groove surface have a reflective coating. Such thin film mirror deposition on the v-groove facet (the film must not be deposited on other fiber surfaces to avoid introducing loss for the pump light in the inner cladding) is so difficult to implement as to make it commercially impractical. In addition, this form of lens-less v-groove coupling does not make it possible to take advantage of the cylindrical lens effect formed by the surface of a round fiber.
In summary, there is a need to increase coupling efficiency of light from pump sources having a large divergence and width into optical waveguides such as double cladding fibers, while reducing the complexity and cost, eliminating low optical damage materials from the optical path of the pump beam, and minimizing the inner cladding diameter to achieve high pump intensity in the fiber, in order to optimize fiber amplifier and laser performance.